The present invention relates to a carburetor for internal combustion engines including a butterfly-type throttle valve, and more particularly to a carburetor having means for preventing the freezing of moisture at a pilot fuel jetting area during a low-load operation of the engine.
In the conventional carburetor of the type described, the throttle valve is partially closed with its periphery near a by-pass port for supplying a pilot fuel during a low-load operation of the engine. When a pilot fuel or a fuel-air mixture is drawn from a pilot fuel or a fuel-air mixture is drawn from a pilot fuel supply system to an intake passage during a low-load operation of the engine in a highly humid and cold atmosphere, the latent heat of evaporation of the fuel causes the freezing of moisture in the intake air to prevent air from flowing through the clearance between the inner surface of the intake passage and the periphery of the throttle valve. As a result, the fuel to air ratio fed to the engine becomes much too rich, thereby impairing the slow-speed operation or the low-load operation of the engine. Otherwise, the freezing of moisture closes the by-pass port so that the fuel-air ratio fed to the engine is made too much lean, resulting in the disorderly engine operation.
In order to prevent the freezing of moisture at the by-pass port or at the clearance between the inner surface of the intake passage and the periphery of the throttle valve, various measures have been proposed.
For example, Japanese utility model application serial No. 50-107,803, laid open to public inspection under No. 52-21,399 issued on Feb. 15, 1977, has disclosed that a heater is employed to heat the portion of the outer surface of the carburetor body at which the throttle valve and by-pass port are located. In this case, however, the heater surrounding the outer surface of the carburetor body is remote from the by-pass port and the inner surface of the intake passage so that they can not be heated to a sufficiently high temperature in a short time.
Another measure has been proposed in Japanese utility model application Ser. No. 50-33,631, laid open to public inspection under Ser. No. 51-114,519 issued on Sept. 17, 1976, in which a heat conductive member is disposed in a pilot fuel passage leading to a pilot fuel port so that one end of the heat conductive member is heated to conduct heat to the by-pass port. Though the heating material is located much nearer to the by-pass port as compared with the aforementioned Japanese utility model application, it takes a considerable time to heat the by-pass port and the inner surface of the intake passage because the heat conductive member itself is not a heater and hence can not serve as a direct heating source.